Transversely-excited film bulk acoustic resonators with interdigital transducer configured to reduce diaphragm stress

ABSTRACT

Acoustic resonators are disclosed. An acoustic resonator includes a substrate having a surface and a single-crystal piezoelectric plate having front and back surfaces. The back surface is attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm spanning a cavity in the substrate. An interdigital transducer (IDT) is formed on the front surface of the piezoelectric plate. The IDT includes: a first busbar and a second busbar disposed on respective portions of the piezoelectric plate other than the diaphragm; a first set of elongate fingers extending from the first bus bar onto the diaphragm; and a second set of elongate fingers extending from the second bus bar onto the diaphragm, the second set of elongate fingers interleaved with the first set of elongate fingers.

RELATED APPLICATION INFORMATION

This patent is a continuation of co-pending U.S. application Ser. No.17/109,011, filed Dec. 1, 2020 entitled TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATORS WITH INTERDIGITAL TRANSDUCER CONFIGURED TO REDUCEDIAPHRAGM STRESS which claims priority from provisional patentapplication 63/021,036, filed May 6, 2020, entitled XBAR WITH NO BUSBARSON DIAPHRAGM, both which are incorporated herein by reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a passband or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies and bandwidths proposed for future communicationsnetworks.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. Radio accesstechnology for mobile telephone networks has been standardized by the3GPP (3^(rd) Generation Partnership Project). Radio access technologyfor 5^(th) generation mobile networks is defined in the 5G NR (newradio) standard. The 5G NR standard defines several new communicationsbands. Two of these new communications bands are n77, which uses thefrequency range from 3300 MHz to 4200 MHz, and n79, which uses thefrequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79use time-division duplexing (TDD), such that a communications deviceoperating in band n77 and/or band n79 uses the same frequencies for bothuplink and downlink transmissions. Bandpass filters for bands n77 andn79 must be capable of handling the transmit power of the communicationsdevice. WiFi bands at 5 GHz and 6 GHz also require high frequency andwide bandwidth. The 5G NR standard also defines millimeter wavecommunication bands with frequencies between 24.25 GHz and 40 GHz.

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is anacoustic resonator structure for use in microwave filters. The XBAR isdescribed in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILMBULK ACOUSTIC RESONATOR. An XBAR resonator comprises an interdigitaltransducer (IDT) formed on a thin floating layer, or diaphragm, of asingle-crystal piezoelectric material. The IDT includes a first set ofparallel fingers, extending from a first busbar and a second set ofparallel fingers extending from a second busbar. The first and secondsets of parallel fingers are interleaved. A microwave signal applied tothe IDT excites a shear primary acoustic wave in the piezoelectricdiaphragm. XBAR resonators provide very high electromechanical couplingand high frequency capability. XBAR resonators may be used in a varietyof RF filters including band-reject filters, band-pass filters,duplexers, and multiplexers. XBARs are well suited for use in filtersfor communications bands with frequencies above 3 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view and two schematic cross-sectional viewsof a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1 .

FIG. 3 is an expanded schematic cross-sectional view of another portionof the XBAR of FIG. 1 .

FIG. 4 is an alternative schematic cross-sectional view of the XBAR ofFIG. 1 .

FIG. 5 is a schematic plan view of another XBAR.

FIG. 6 is a graphic illustrating a shear primary acoustic mode in anXBAR.

FIG. 7 is a schematic block diagram of a filter using XBARs.

FIG. 8 is a flow chart of a method of fabricating an XBAR.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100. XBAR resonators such as the resonator 100 may beused in a variety of RF filters including band-reject filters, band-passfilters, duplexers, and multiplexers. XBARs are well suited for use infilters for communications bands with frequencies above 3 GHz.

The XBAR 100 includes a piezoelectric plate 110 having a front surface112 and a back surface 114. The front and back surfaces are essentiallyparallel. “Essentially parallel” means parallel to the extent possiblewithin normal manufacturing tolerances. The piezoelectric plate is athin single-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented in this patent, the piezoelectric plates are rotated YX-cut.However, XBARs may be fabricated on piezoelectric plates with othercrystallographic orientations including Z-cut, rotated Z-cut, androtated Y-cut.

The back surface 114 of the piezoelectric plate 110 is attached to asurface 122 of a substrate 120 except for a portion of the piezoelectricplate 110 that forms a diaphragm 115 spanning a cavity 140 formed in thesubstrate 120. The portion of the piezoelectric plate that spans thecavity is referred to herein as the “diaphragm” due to its physicalresemblance to the diaphragm of a microphone. As shown in FIG. 1 , thediaphragm 115 is contiguous with the rest of the piezoelectric plate 110around all of a perimeter 145 of the cavity 140. In this context,“contiguous” means “continuously connected without any interveningitem”.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be attached to the substrate 120using a wafer bonding process. Alternatively, the piezoelectric plate110 may be grown on the substrate 120 or otherwise attached to thesubstrate. The piezoelectric plate 110 may be attached directly to thesubstrate 120 or may be attached to the substrate 120 via one or moreintermediate material layers (not shown).

The cavity 140 is an empty space within a solid body of the resonator100. The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120 (as shown subsequently in FIG. 3 ). The cavity 140 may be formed,for example, by selective etching of the substrate 120 before or afterthe piezoelectric plate 110 and the substrate 120 are attached.

Two or more conductor levels are formed on the front surface 112 of thepiezoelectric plate. The two or more conductor levels are patternedusing different photomasks such that the two or more conductor levelsare not identical. Each conductor level is formed of one or more thinmetal layers. The first conductor level includes an interdigitaltransducer (IDT) 130. An IDT is an electrode structure for convertingbetween electrical and acoustic energy in piezoelectric devices. The IDT130 includes a first plurality of parallel elongated conductors,commonly called “fingers”, such as finger 136, extending from a firstlevel of a first busbar 132. The IDT 130 includes a second plurality offingers extending from a first level of a second busbar 134. The firstand second pluralities of parallel fingers are interleaved. Theinterleaved IDT fingers overlap for a distance AP, commonly referred toas the “aperture” of the IDT. The center-to-center distance L betweenthe outermost fingers of the IDT 130 is the “length” of the IDT.

The term “busbar” refers to the conductors that interconnect the firstand second sets of fingers in an IDT. As shown in FIG. 1 , each firstbusbar level 132, 134 is an elongated rectangular conductor with a longaxis orthogonal to the interleaved IDT fingers and having a lengthapproximately equal to the length L of the IDT. The busbars of an IDTneed not be rectangular or orthogonal to the interleaved fingers and mayhave lengths longer than the length of the IDT.

The first conductor level includes the interleaved fingers of the IDTand the first level of the busbars 132, 134. In a filter, the firstconductor level may also be a first level of other conductorsinterconnecting multiple XBARs in a filter circuit.

A second conductor level may include second busbar levels 152, 154formed over all or portions of the first busbar levels 132, 134,respectively. The second conductor level is typically thicker than thefirst conductor level. The second conductor level improves theelectrical and thermal conductivity of the busbars and other conductors.In a filter, the second conductor level may also form a second level ofother conductors interconnecting multiple XBARs in a filter circuit.

The first and second busbars 132/152, 134/154 serve as the terminals ofthe XBAR 100. A radio frequency or microwave signal applied to the IDT,which is to say applied between the first and second busbars 132/152,134/154, excites a primary acoustic mode within the piezoelectric plate110. As will be discussed in further detail, the primary acoustic modeis a bulk shear mode where acoustic energy propagates along a directionsubstantially orthogonal to the surface of the piezoelectric plate 110,which is also normal, or transverse, to the direction of the electricfield created by the IDT fingers. Thus, the XBAR is considered atransversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that thefingers, such as finger 136, of the IDT 130 are disposed on thediaphragm 115 of the piezoelectric plate that spans, or is suspendedover, the cavity 140. The first and second busbars 132/152, 134/154 arenot on the diaphragm. Placing the first and second busbars 132/152,134/154 off the diaphragm reduces stress in the diaphragm. As shown inFIG. 1 , the cavity 140 has a rectangular shape with an extent greaterthan the aperture AP and length L of the IDT 130. A cavity of an XBARmay have a different shape, such as a regular or irregular polygon. Thecavity of an XBAR may more or fewer than four sides, which may bestraight or curved.

For ease of presentation in FIG. 1 , the geometric pitch and width ofthe IDT fingers is greatly exaggerated with respect to the length(dimension L) and aperture (dimension AP) of the XBAR. An XBAR for a 5Gdevice will have an IDT with more than ten parallel fingers. An XBAR mayhave hundreds, possibly thousands, of parallel fingers in the IDT.Similarly, the thickness of all elements is greatly exaggerated in thein the cross-sectional views.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100.The portion of the XBAR 100 shown in FIG. 2 is identified in FIG. 1 as“DETAIL C”. The piezoelectric plate 110 is a single-crystal layer ofpiezoelectrical material having a thickness ts. ts may be, for example,100 nm to 1500 nm. When used in filters for bands from 3.4 GHZ to 6 GHz(e.g. bands n77, n79, 5 GHz Wi-Fi™, 6 GHz Wi-Fi™), the thickness ts maybe, for example, 200 nm to 1000 nm.

A front-side dielectric layer 214 may be formed on the front side of thepiezoelectric plate 110. The “front side” of the XBAR is the surfacefacing away from the substrate. The front-side dielectric layer 214 hasa thickness tfd. The front-side dielectric layer 214 is formed betweenthe IDT fingers 238. Although not shown in FIG. 2 , the front-sidedielectric layer 214 may also be deposited over the IDT fingers 238. Aback-side dielectric layer 216 may be formed on the back side of thepiezoelectric plate 110. The back-side dielectric layer 216 has athickness tbd. The front-side and back-side dielectric layers 214, 216may be a non-piezoelectric dielectric material, such as silicon dioxideor silicon nitride. tfd and tbd may be, for example, 0 to 500 nm. tfdand tbd are typically less than the thickness ts of the piezoelectricplate. tfd and tbd are not necessarily equal, and the front-side andback-side dielectric layers 214, 216 are not necessarily the samematerial. Either or both of the front-side and back-side dielectriclayers 214, 216 may be formed of multiple layers of two or morematerials.

The IDT fingers 238 are portions of the first conductor level. The firstconductor level may be one or more layers of aluminum, a substantiallyaluminum alloy, copper, a substantially copper alloy, chromium,platinum, beryllium, gold, molybdenum, tungsten, or some otherconductive material. Thin (relative to the total thickness of theconductors) layers of metals such as chromium or titanium may be formedunder and/or over the fingers to improve adhesion between the fingersand the piezoelectric plate 110 and/or to passivate or encapsulate thefingers. The second conductor level on the busbars (152, 154 in FIG. 1 )of the IDT may be made of the same or different materials as thefingers. As shown in FIG. 2 , the IDT fingers 238 have rectangularcross-sections. The IDT fingers may have some other cross-sectionalshape.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the acoustic wavelength at the resonancefrequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDTis typically close to 0.5 (i.e., the mark or finger width is aboutone-fourth of the acoustic wavelength at resonance). In an XBAR, thepitch p of the IDT is typically 2 to 20 times the width w of thefingers. In addition, the pitch p of the IDT is typically 2 to 20 timesthe thickness is of the piezoelectric slab 110. The width of the IDTfingers in an XBAR is not constrained to one-fourth of the acousticwavelength at resonance. For example, the width of XBAR IDT fingers maybe 500 nm or greater, such that the IDT can be fabricated using opticallithography. The thickness tc1 of the IDT fingers may be from 100 nm toabout equal to the width w. The thickness of the second conductor levelon the busbars (152, 154 in FIG. 1 ) of the IDT may be the same as, orgreater than, the thickness tm of the IDT fingers.

FIG. 3 shows another detailed schematic cross-sectional view of the XBAR100. The portion of the XBAR 100 shown in FIG. 3 is identified in FIG. 1as “DETAIL D”. The piezoelectric plate 110 is attached to and supportedby the substrate 120. A portion of the piezoelectric plate forms adiaphragm extending over the cavity 140 in the substrate 120. A firstbusbar level 134 is a portion of a first conductor level havingthickness tc1. A second busbar level 154 is a portion of a secondconductor level having thickness tc2. tc2 is typically greater than tc1.The edges of the first and second conductor levels and the side of thecavity 140 are not necessarily perpendicular to the surfaces of thepiezoelectric plate, as shown in FIG. 3 .

An intersection of the first busbar level 134 and the front surface 112of the piezoelectric plate 110 defines a first busbar edge 334 proximatethe cavity 140. An intersection of the cavity 140 and the back surface114 of the piezoelectric plate 110 defines a cavity edge 340 proximatethe first busbar. The first busbar edge 334 is offset from the proximatecavity edge 340 by a distance do1 in a direction parallel to thesurfaces of the piezoelectric plate 110. do1 is greater than or equal tozero. The distance do1 may be as small as allowed by expectedmanufacturing tolerances. For example, the nominal value of do1 may beset to the worst-case expected misregistration between the cavity 140and the photomask used to pattern the first conductor level to ensurethat, even with the worst-case misregistration, do1 will be greater thanor equal to zero. A second busbar edge proximate the cavity and a cavityedge proximate the second busbar may be similarly defined.

The second busbar level 154 may cover a portion or substantially all ofthe first busbar level 134. In this context, the phrase “substantiallyall” means as close to all as allowed by expected manufacturingtolerances. For example, a nominal offset do2 between any edge of thesecond busbar level 154 and a proximate edge of the first busbar level134 may be set to the worst-case expected misregistration between thephotomasks used to pattern the first and second conductor levels.

FIG. 4 is an alternative cross-sectional view along the section planeA-A defined in FIG. 1 . In FIG. 4 , a piezoelectric plate 410 isattached to a substrate 420. A portion of the piezoelectric plate 410forms a diaphragm 415 spanning a cavity 440 in the substrate. The cavity440 does not fully penetrate the substrate 420. Fingers of an IDT aredisposed on the diaphragm 415. The cavity 440 may be formed, forexample, by etching the substrate 420 before attaching the piezoelectricplate 410. Alternatively, the cavity 440 may be formed by etching thesubstrate 420 with a selective etchant that reaches the substratethrough one or more openings (not shown) provided in the piezoelectricplate 410. In this case, the diaphragm 415 may be contiguous with therest of the piezoelectric plate 410 around a large portion of aperimeter of the cavity 440. For example, the diaphragm 415 may becontiguous with the rest of the piezoelectric plate 410 around at least50% of the perimeter of the cavity 440.

FIG. 5 is a plan view of another XBAR 500. The XBAR 500 includes apiezoelectric plate 510 attached to a substrate that is not visible. Aportion of the piezoelectric plate 510 forms a diaphragm spanning acavity in the substrate. The dashed line 545 is a perimeter of thecavity defined by the intersection of the cavity and the back surface(i.e. the surface facing away from the viewer) of the piezoelectricplate 510. The portion of the piezoelectric plate 510 within the dashedrectangle 545 is the diaphragm.

An IDT 530 is formed on a front surface (i.e. the surface facing theviewer) of the piezoelectric plate. The IDT 530 includes a firstplurality of parallel elongated conductors, commonly called “fingers”,such as finger 536, extending from a first busbar 532. The IDT 530includes a second plurality of fingers extending from a second busbar534. The first and second pluralities of parallel fingers areinterleaved. The interleaved IDT fingers overlap for a distance AP,commonly referred to as the “aperture” of the IDT. The first and secondbusbars 532, 534 are not on the diaphragm. The interleaved fingersextend from their respective busbars onto the diaphragm such that theentire aperture of the IDT is on the diaphragm. The IDT 530 alsoincludes dummy fingers, such as dummy finger 538, that extend from thebusbars onto the diaphragm. The term “dummy fingers” is commonly used todenote short fingers extending from the busbars of an IDT between thelonger interleaved fingers. The presence of dummy fingers, such as dummyfinger 538, extending from the busbars 532, 534 onto the diaphragm mayhelp with removing heat from the diaphragm.

The interleaved fingers, the dummy fingers, and the busbars 532, 534 ofthe IDT 530 are formed from a first level conductor level. A secondconductor level 552, 554 may overlap all or portions of the first andsecond busbars 532, 534, respectively.

FIG. 6 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 6 shows a small portion of an XBAR 600including a piezoelectric plate 610 and three interleaved IDT fingers630. A radio frequency (RF) voltage is applied to the interleavedfingers 630. This voltage creates a time-varying electric field betweenthe fingers. The direction of the electric field is primarily lateral,or parallel to the surface of the piezoelectric plate 610, as indicatedby the arrows labeled “electric field”. Since the dielectric constant ofthe piezoelectric plate is significantly higher than the surroundingair, the electric field is highly concentrated in the plate relative tothe air. The lateral electric field introduces shear deformation, andthus strongly excites a shear-mode acoustic mode, in the piezoelectricplate 610. Shear deformation is deformation in which parallel planes ina material remain parallel and maintain a constant distance whiletranslating relative to each other. A “shear acoustic mode” is anacoustic vibration mode in a medium that results in shear deformation ofthe medium. The shear deformations in the XBAR 600 are represented bythe curves 660, with the adjacent small arrows providing a schematicindication of the direction and magnitude of atomic motion. The degreeof atomic motion, as well as the thickness of the piezoelectric plate610, have been greatly exaggerated for ease of visualization. While theatomic motions are predominantly lateral (i.e. horizontal as shown inFIG. 6 ), the direction of acoustic energy flow of the excited primaryshear acoustic mode is substantially orthogonal to the surface of thepiezoelectric plate, as indicated by the arrow 665.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. In addition, the piezoelectric couplingfor shear wave XBAR resonances can be high (>20%) compared to otheracoustic resonators. High piezoelectric coupling enables the design andimplementation of microwave and millimeter-wave filters with appreciablebandwidth.

FIG. 7 is a schematic circuit diagram and layout for a high frequencyband-pass filter 700 using XBARs. The filter 700 has a conventionalladder filter architecture including four series resonators 710A, 710B,710C, 710D and three shunt resonators 720A, 720B, 720C. The seriesresonators 710A, 710B, 710C, and 710D are connected in series between afirst port and a second port (hence the term “series resonator”). InFIG. 7 , the first and second ports are labeled “In” and “Out”,respectively. However, the filter 700 is bidirectional and either portmay serve as the input or output of the filter. The shunt resonators720A, 720B, 720C are connected from nodes between the series resonatorsto ground. A filter may contain additional reactive components, such asinductors, not shown in FIG. 7 . All the shunt resonators and seriesresonators are XBARs. The inclusion of four series and three shuntresonators is exemplary. A filter may have more or fewer than seventotal resonators, more or fewer than four series resonators, and more orfewer than three shunt resonators. Typically, all of the seriesresonators are connected in series between an input and an output of thefilter. All of the shunt resonators are typically connected betweenground and the input, the output, or a node between two seriesresonators.

In the exemplary filter 700, the series resonators 710A, B, C, D and theshunt resonators 720A, B, D of the filter 700 are formed on a singleplate 730 of piezoelectric material bonded to a silicon substrate (notvisible). Each resonator includes a respective IDT (not shown), with atleast the fingers of the IDT disposed over a cavity in the substrate. Inthis and similar contexts, the term “respective” means “relating thingseach to each”, which is to say with a one-to-one correspondence.

Each of the resonators 710A, 710B, 710C, 710D, 720A, 720B, 720C in thefilter 700 has a resonance where the admittance of the resonator is veryhigh and an anti-resonance where the admittance of the resonator is verylow. The resonance and anti-resonance occur at a resonance frequency andan anti-resonance frequency, respectively, which may be the same ordifferent for the various resonators in the filter 700. Inover-simplified terms, each resonator can be considered a short-circuitat its resonance frequency and an open circuit at its anti-resonancefrequency. The input-output transfer function will be near zero at theresonance frequencies of the shunt resonators and at the anti-resonancefrequencies of the series resonators. In a typical filter, the resonancefrequencies of the shunt resonators are positioned below the lower edgeof the filter's passband and the anti-resonance frequencies of theseries resonators are position above the upper edge of the passband.

Description of Methods

FIG. 8 is a simplified flow chart showing a process 800 for making anXBAR or a filter incorporating XBARs. The process 800 starts at 805 witha substrate and a plate of piezoelectric material and ends at 895 with acompleted XBAR or filter. The flow chart of FIG. 8 includes only majorprocess steps. Various conventional process steps (e.g. surfacepreparation, cleaning, inspection, baking, annealing, monitoring,testing, etc.) may be performed before, between, after, and during thesteps shown in FIG. 8 .

The flow chart of FIG. 8 captures three variations of the process 800for making an XBAR which differ in when and how cavities are formed inthe substrate. The cavities may be formed at steps 810A, 810B, or 810C.Only one of these steps is performed in each of the three variations ofthe process 800.

The piezoelectric plate may be lithium niobate or lithium tantalate. Thepiezoelectric plate may be Z-cut, rotated Z-cut, or rotated YX-cut, orsome other cut. The substrate may preferably be silicon. The substratemay be some other material that allows formation of deep cavities byetching or other processing.

In one variation of the process 800, one or more cavities are formed inthe substrate at 810A, before the piezoelectric plate is bonded to thesubstrate at 820. The cavity may be formed at 810A before or after theback-side dielectric layer is formed at 815. A separate cavity may beformed for each resonator in a filter device. The one or more cavitiesmay be formed using conventional photolithographic and etchingtechniques. Typically, the cavities formed at 810A will not penetratethrough the substrate, and the resulting resonator devices will have across-section as shown in FIG. 3 .

At 815, the back-side dielectric layer may be formed by depositingsilicon dioxide or another dielectric material on the back surface ofthe piezoelectric plate, the surface of the substrate, or both.

At 820, the piezoelectric plate is bonded to the substrate such that theback-side dielectric layer is sandwiched between the substrate and thepiezoelectric plate. The piezoelectric plate and the substrate may bebonded by a wafer bonding process. Typically, the mating surfaces of thesubstrate and the piezoelectric plate are highly polished. Anintermediate material, which may be the back-side dielectric layer, maybe formed or deposited on the mating surface of one or both of thepiezoelectric plate and the substrate. One or both mating surfaces maybe activated using, for example, a plasma process. The mating surfacesmay then be pressed together with considerable force to establishmolecular bonds between the piezoelectric plate and the substrate orintermediate material layers.

A conductor pattern, including IDTs of each XBAR, is formed at 830 bydepositing and patterning one or more conductor layer on the front-sideof the piezoelectric plate. The conductor layer may be, for example,aluminum, an aluminum alloy, copper, a copper alloy, or some otherconductive metal. Optionally, one or more layers of other materials maybe disposed below (i.e. between the conductor layer and thepiezoelectric plate) and/or on top of the conductor layer. For example,a thin film of titanium, chrome, or other metal may be used to improvethe adhesion between the conductor layer and the piezoelectric plate. Aconduction enhancement layer of gold, aluminum, copper or other higherconductivity metal may be formed over portions of the conductor pattern(for example the IDT bus bars and interconnections between the IDTs).

At 840, a front-side dielectric layer may be formed by depositing one ormore layers of dielectric material on the front-side of thepiezoelectric plate. The one or more dielectric layers may be depositedusing a conventional deposition technique such as sputtering,evaporation, or chemical vapor deposition. The one or more dielectriclayers may be deposited over the entire surface of the piezoelectricplate. Alternatively, one or more lithography processes (usingphotomasks) may be used to limit the deposition of the dielectric layersto selected areas of the piezoelectric plate, such as only between theinterleaved fingers of the IDTs. Masks may also be used to allowdeposition of different thicknesses of a frequency setting dielectriclayer on different portions of the piezoelectric plate corresponding todifferent shunt resonators.

The actions at 830 and 840 may be performed in either order or may beperformed as a single integrated sequence of process steps. For example,the front-side dielectric layer may be deposited over the front surfaceof the piezoelectric plate. The front-side dielectric layer may then becoated with photoresist. The photoresist may then be exposed through amask to define the areas where the IDT fingers and other conductors willbe formed. The front-side dielectric layer may then be etched to removethe dielectric material from the areas where the IDT fingers and otherconductors will be formed. The conductor metal can then be deposited tocover the remaining photoresist and fill the areas where the front-sidedielectric layer was removed. The photoresist can then be stripped,lifting off the excess conductor metal and leaving the IDT fingers inthe grooves previously etched in the front-side dielectric layer. Theconductor pattern and the front-side dielectric layer may be formedusing some other sequence of process steps.

In a second variation of the process 800, one or more cavities areformed in the back-side of the substrate at 810B. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back-side of the substrate to thepiezoelectric plate. In this case, the resulting resonator devices willhave a cross-section as shown in FIG. 1 .

In a third variation of the process 800, one or more cavities in theform of recesses in the substrate may be formed at 810C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. A separate cavity may be formed for each resonatorin a filter device. The one or more cavities formed at 810C will notpenetrate through the substrate, and the resulting resonator deviceswill have a cross-section as shown in FIG. 3 .

In all variations of the process 800, the filter device is completed at860. Actions that may occur at 860 include depositing anencapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;excising individual devices from a wafer containing multiple devices;other packaging steps; and testing. Another action that may occur at 860is to tune the resonant frequencies of the resonators within the deviceby adding or removing metal or dielectric material from the front-sideof the device. This tuning may also include selectively removingmaterial from shunt resonators to create multiple frequency settingdielectric layer thicknesses. After the filter device is completed, theprocess ends at 895.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic resonator device comprising: a substratehaving a surface; a single-crystal piezoelectric plate having front andback surfaces, the back surface attached to the surface of the substrateexcept for a portion of the piezoelectric plate forming a diaphragmspanning a cavity in the substrate; and an interdigital transducer (IDT)formed on the front surface of the piezoelectric plate, the IDTincluding: a first busbar and a second busbar disposed on respectiveportions of the piezoelectric plate other than the diaphragm, a firstset of elongate fingers extending from the first bus bar onto thediaphragm, a second set of elongate fingers extending from the secondbus bar onto the diaphragm, the second set of elongate fingersinterleaved with the first set of elongate fingers, wherein thepiezoelectric plate and the first and second sets of elongate fingersare configured such that a radio frequency signal applied between thefirst and second busbars excites a primary shear acoustic mode in thediaphragm.
 2. The acoustic resonator device of claim 1, furthercomprising: a first set of dummy fingers extending from the first busbaronto the diaphragm, each of the first set of dummy fingers aligned witha corresponding one of the second set of elongate fingers; and a secondset of dummy fingers extending from the second busbar onto thediaphragm, each of the second set of dummy fingers aligned with acorresponding one of the first set of elongate fingers.
 3. The acousticresonator device of claim 1, further comprising: a second conductorlevel formed over the front surface of the piezoelectric plate, thesecond conductor level covering at least portions of the first andsecond busbars.
 4. The acoustic resonator device of claim 3, wherein thesecond conductor level covers substantially all of the first and secondbusbars.
 5. The acoustic resonator device of claim 1, wherein the IDT ispart of a conductor pattern.
 6. The acoustic resonator device of claim5, wherein the piezoelectric plate is one of lithium niobate and lithiumtantalate.
 7. The acoustic resonator device of claim 6, wherein thepiezoelectric plate is one of Z-cut, rotated Z-cut, and rotated YX-cut.8. An acoustic resonator device comprising: a substrate having asurface; a single-crystal piezoelectric plate having front and backsurfaces, the back surface attached to the surface of the substrateexcept for a portion of the piezoelectric plate forming a diaphragmspanning a cavity in the substrate; and a first conductor level formedon the front surface of the piezoelectric plate, the first conductorlevel including: first and second busbars; a first set of elongatefingers extending from the first bus bar, a second set of elongatefingers extending from the second bus bar, the second set of elongatefingers interleaved with the first set of elongate fingers, wherein thefirst and second busbars are not on the diaphragm, and the first andsecond sets of elongate fingers extend from the respective busbars ontothe diaphragm, and wherein one of the first distance and the seconddistance are as small as allowed by expected manufacturing tolerances,or nominal values of the first distance and the second distance areequal to a worst-case expected misregistration between the cavity and aphotomask used to pattern the first conductor level.
 9. The acousticresonator device of claim 8, wherein a first distance, parallel to thefront surface of the piezoelectric plate, from an edge of the firstbusbar proximate the cavity to an edge of the cavity proximate the firstbusbar is greater than or equal to zero, and a second distance, parallelto the front surface of the piezoelectric plate, from an edge of thesecond busbar proximate the cavity to an edge of the cavity proximatethe second busbar is greater than or equal to zero.
 10. The acousticresonator device of claim 8, further comprising: a first set of dummyfingers extending from the first busbar onto the diaphragm, each of thefirst set of dummy fingers aligned with a corresponding one of thesecond set of elongate fingers; and a second set of dummy fingersextending from the second busbar onto the diaphragm, each of the secondset of dummy fingers aligned with a corresponding one of the first setof elongate fingers.
 11. The acoustic resonator device of claim 8,further comprising: a second conductor level formed over the frontsurface of the piezoelectric plate, the second conductor level coveringat least portions of the first and second busbars.
 12. The acousticresonator device of claim 11, wherein the second conductor level coverssubstantially all of the first and second busbars.
 13. An acousticresonator device comprising: a substrate having a surface; asingle-crystal piezoelectric plate having front and back surfaces, theback surface attached to the surface of the substrate except for aportion of the piezoelectric plate forming a diaphragm spanning a cavityin the substrate; and a first conductor level formed on the frontsurface of the piezoelectric plate, the first conductor level including:first and second busbars; a first set of elongate fingers extending fromthe first bus bar, a second set of elongate fingers extending from thesecond bus bar, the second set of elongate fingers interleaved with thefirst set of elongate fingers, wherein the first and second busbars arenot on the diaphragm, wherein the first and second sets of elongatefingers extend from the respective busbars onto the diaphragm, andwherein the piezoelectric plate and the first and second sets ofelongate fingers are configured such that a radio frequency signalapplied between the first and second busbars excites a primary shearacoustic mode in the diaphragm.
 14. The acoustic resonator device ofclaim 13, wherein the piezoelectric plate is one of lithium niobate andlithium tantalate.
 15. The acoustic resonator device of claim 14,wherein the piezoelectric plate is one of Z-cut, rotated Z-cut, androtated YX-cut.
 16. An acoustic resonator device comprising: a substratehaving a surface; a single-crystal piezoelectric plate having front andback surfaces, the back surface attached to the surface of the substrateexcept for a portion of the piezoelectric plate forming a diaphragmspanning a cavity in the substrate; first and second busbars formed onthe front surface of the piezoelectric plate; a first set of elongatefingers extending from the first bus bar onto the diaphragm, a secondset of elongate fingers extending from the second bus bar onto thediaphragm, the second set of elongate fingers interleaved with the firstset of elongate fingers; a first set of dummy fingers extending from thefirst busbar onto the diaphragm, each of the first set of dummy fingersaligned with a corresponding one of the second set of elongate fingers;and a second set of dummy fingers extending from the second busbar ontothe diaphragm, each of the second set of dummy fingers aligned with acorresponding one of the first set of elongate fingers, wherein theentire first bus bar and the entire second bus bar are not on thediaphragm.
 17. The acoustic resonator device of claim 16, wherein thefirst and second sets of dummy fingers are shorter than the first andsecond sets of elongate fingers.
 18. The acoustic resonator device ofclaim 16, further comprising: a second conductor level formed over thefront surface of the piezoelectric plate, the second conductor levelcovering at least portions of the first and second busbars.
 19. Theacoustic resonator device of claim 18, wherein the second conductorlevel covers substantially all of the first and second busbars.
 20. Theacoustic resonator device of claim 16, wherein one of The piezoelectricplate is one of lithium niobate and lithium tantalate, or thepiezoelectric plate is one of Z-cut, rotated Z-cut, and rotated YX-cut.